Verifi Concrete Quality Blog

Posts Tagged ‘slump measurement’

Is ‘Initial Slump’ or ‘Leave Plant Slump’ More Useful?

by Verifi

Verifi reports indicate the initial slump and leave plant slump. Initial slump is the first valid slump determined by Verifi after post-batch mixing is complete. Leave plant slump is determined when Verifi GPS shows the truck has left the boundary of the plant.

Here’s how both can be used.

 ©Verifi LLC

How Verifi Changes Things

Without Verifi, drivers typically adjust the slump in the plant to slightly above target. The hope is for the slump to be relatively close to target upon reaching the jobsite.

Verifi adjusts slump in transit; therefore, it is no longer necessary to batch above the target slump. Verifi recommends batching at or slightly below the target slump.  Drivers can load, mix, and go, then let Verifi adjust slump in transit up to the target slump.   This can save significant time in the plant.

Leave Plant Slump

The reason to check the leave plant slump with Verifi is to ensure drivers do not leave with a load batched significantly above target and likely to be rejected at the jobsite.

Otherwise, the concrete does not care where it is, so the slump at leave plant is not relevant to concrete quality.

Initial Slump

The initial slump can be used to assess how closely slump is batched to target. Variations in initial slump can suggest poor batching, a change in aggregate moisture, or a change in materials. This feedback can be highly useful for batching subsequent loads closer to target and identifying needed mix design changes.

In Conclusion

Without Verifi, both initial and leave plant slump are important. With Verifi, initial slump is much more useful that leave plant slump.

How Accurate is the Slump Test?

by Verifi

Concrete can be accepted or rejected on the basis of slump.  NRMCA estimates that 43% of concrete rejected at the jobsite is due to slump.  So, how accurate is this test?

How much should two tests vary?

According to ASTM C143, when the same person is testing the same concrete, results from two “properly conducted” tests can vary as much as 1.13 in. for a concrete with a slump of 6.5 in.  When two people perform the test, the range can be as high as 1.5 in.


Slump test precision and bias

Slump Test Precision and Bias in ASTM C143

(click for larger image)


Is the concrete sampled properly?

Further variation can be introduced when sampling from the truck to the wheelbarrow to the slump cone.  ASTM C172 requires that concrete be sampled from the middle portion of the discharge.  When evaluating concrete for acceptance, ASTM C94 allows samples for slump and air to be taken after ¼ yd3 or ¼ m3 is discharged.  The first portion of concrete can misrepresent the remaining portion if the concrete is not fully mixed, segregated, or the top of the drum and chute are not clean and damp.


What about improperly conducted tests?

The number and vigor of stokes for compaction (3 layers, 25 strokes per layer), the time to perform the test (less than 2.5 min), and the time to lift the cone (5+-/2 s) all affect test results.  The test should be conducted on a flat, rigid, level, and moist surface.


Is slump even applicable?

ASTM C143 indicates that slump may not be applicable for concrete with slump less than ½ in. or greater than 9 in.  If a portion of the concrete shears off from the sample, the test is considered not applicable.


How does Verifi help?

Verifi automates the determination of slump by using sophisticated algorithms to process hydraulic pressure and drum speed sensor data from the truck. See how.

Verifi determines slump

These measurements are made on the full load in the drum to avoid sampling errors.  Human error in performing the test is eliminated.  Verifi  typically measures slump 95% of the time within ASTM limits.   Much of this difference is due to sampling and testing errors with the manual slump test.